Many vehicles employ step ratio transmissions to transfer power between an internal combustion engine and the vehicle wheels. Step ratio transmissions provide a discrete set of speed ratios between the engine and the wheels. In an automatic transmission, a controller selects the transmission ratio in response to the vehicle speed and a driver demand, usually communicated by depressing an accelerator pedal. High speed, low demand situations call for a low speed ratio such that the engine operates most efficiently. Low speed, high demand situations call for a high speed ratio because so the engine operates at the speed at which it can generate the most power. As the conditions change, the transmission must shift between speed ratios. Vehicle occupants may feel these shifts and may be annoyed if these shifts occur too frequently.
In a modular hybrid transmission (MHT) architecture, the vehicle also has a traction motor connected at the input of the transmission. The traction motor is electrically connected to a battery. The motor may be used in either a motoring mode in which energy from the battery is used to supplement the engine power or in a generating mode in which the motor converts mechanical energy into electrical energy which is stored in the battery.